GZ-34 filter cap value mistake??

[rudedogg]

Hey guys, I just ordered new filter caps for an Eico hf-50 mono amp I’m working on. The original twin multi-section cans are two sections each, both are 20/20uf @ 500v. I ordered two F&T cans that are 32/32uf @ 500v. The problem is that I then looked closer at the schematic, and it appears the first two cap sections after the rectifier (c1 a&b ) are tied together in parallel! Oops! Sooo now I will have a 64ufd cap for the GZ34 instead of the 40uf. How bad of a condition is this? I can call AES tomorrow morning and cancel the caps, or will the rectifier survive my error. Or maybe I can just use one 32uf section?
Help! Lol

 

[embrown057]

They will be fine. I like the F&T caps for valve rebuilds. Often these builds used minimal filtering and can benefit from increased capacitance.

 

[Dave451]

The datasheets specify 60 uF max for input filter capacity, so you should be OK.

 

[rudedogg]

The datasheets specify 60 uF max for input filter capacity, so you should be OK.

Thanks guys. I guess i’ll go ahead and see how it works.

 

[primosounds]

Thanks guys. I guess i’ll go ahead and see how it works.

I think this kinda depends on the brand of 5ar4 you are using. If it is a genuine strong Mullard , i would think you would have no problem. If it is a currently made 5ar4 , I would be somewhat worried about that rectifier arcing. Maybe not the first few times but my experiences with the currently made 5ar4 is not very pleasant.

 

[billyz]

Or put a resistor between the sections.
And have it all.

 

[rudedogg]

I think this kinda depends on the brand of 5ar4 you are using. If it is a genuine strong Mullard , i would think you would have no problem. If it is a currently made 5ar4 , I would be somewhat worried about that rectifier arcing.

It is an old metal base Mullard. Probably original to the amp. I am worried about hurting that old tube, but reading thru other forums on the web, it seems the guitar amp guys put 100uf first section filters on that tube!

 

[nj pheonix]

I'm guessing here, not familiar with this but based on era, I bet this would benefit from a bucking transformer.
(Well covered, old line voltage tended to be 115vac or there abouts , modern power tends to be 122 or so).
If you buck, it drops surge (and peak) voltages.
It also should bring back in line with stock voltages (B+ and filament).
Which helps your cause as well.

 

[rudedogg]

Or put a resistor between the sections.
And have it all.

Would splitting the sections with a resistor save the rectifier inrush current? I have tons of NOS Vishay wire wound resistors of 5-10 watts....

 

[trobbins]

If you are keen, you can get a better understanding by setting up your amp power supply in PSUD2 and changing part values to see what happens to the initial current waveform peak, and to the waveform peak when everything has settled down - they are the datasheet values that tell you if you are exceeding the valve rating. You will have to be keen, as you will need to measure the power transformer winding resistances, and learn to run a new PC program.

 

[nj pheonix]

The datasheets specify 60 uF max for input filter capacity, so you should be OK.

Probably on your side in this scenario is that these caps generally are spec'd +/- 20% so you should easily fall within this

 

[gadget73]

Also consider thats 60uf max with all other conditions at max. You can eek by with a wee bit more if the rest of the conditions are below that.

 

[peterh]

To be on the safe side : connect the second 32uF cap in parallell with C2A and after R24. This
might do more hum reduction then both caps as C1A and C1B. And it's easy to test
both positions to figure out the least hum.

 

[primosounds]

Would splitting the sections with a resistor save the rectifier inrush current? I have tons of NOS Vishay wire wound resistors of 5-10 watts....

The amount of inrush current depends on the size of the first cap. So, using a smaller capacitance is less current inrush. If thiis was my amp i would break up the double cap into 32 for the input then add a choke and use the second section with additional capacitance. If you know your B+ with the original PS you can adjust the size of the choke to reduce the voltage to hit your desired B+. A choke will give you better voltage regulation vs a resistor. EICO, i think tried to save money by not using them.

 

[rudedogg]

Wow! A lot of good advice and suggestions here, Thankyou all. Well, I did not cancel the F&T cans so I should have all the parts for restoration soon. Except for a power switch..... which I forgot!

 

[rudedogg]

So i have installed the first 32/32 can in place of the 20/20 original can. For the time being, i did not connect the second section of the cap. Thus i have only 32uf instead of the original 40 with the two 20uf sections tied together. I can finish the amp up and see if i get any hum, but do you think i will do any harm by running it this way with 20% less filtration after the rectifier?
John

 

[embrown057]

You should have connected the second section, your just wasting you time under filtering that section. Your not going to stress anything adding that nominal value. It's common knowledge most of these older designs were under filtered.

 

[rudedogg]

You should have connected the second section, your just wasting you time under filtering that section. Your not going to stress anything adding that nominal value. It's common knowledge most of these older designs were under filtered.

Maybe so, and I can easily just jumper the two sections together with a 3/4” piece of wire if necessary. I just don’t want to flash this old tube, probably as valuable as the amp itself! Lol

 

[embrown057]

Trade off for lower ripple value VS your NOS valve. If that valve is in good shape you would be fine. Then again is the valve is marginal would not you want to use a better rectifier valve?

 

[rudedogg]

Trade off for lower ripple value VS your NOS valve. If that valve is in good shape you would be fine. Then again is the valve is marginal would not you want to use a better rectifier valve?

Well, I am recapping the amp for a friend. It’s all original, tubes are all 1958 Mullards and Bugle Boy. I need to test the el-34s yet. The old Mullard metal base rectifiers are supposed to be great, but it’s been around awhile. I did not realize the first two sections of the filter cap were parallel until after I ordered the new 32/32 cans. I thought the amp was really under filtered at 20uf first section! I may connect them together after I install an ntc device on the line in of the tranny. I think then I can safely push the envelope a little with that can section. What do you think of that option, or adding a dropping resistor between the two sections?